There's a Suzuki shop in Vancouver BC (sorry, don't recall which one) that has cut earlier versions of Bandit tanks in two (horizontally) and welded in a strip of metal to increase capacity. Several years ago there was a member here who had one of their modded tanks on his 1G and he posted about it. I don't recall if which version of the Alley he had posted that on, so you may or may not be able to find it here. I do recall the end result looked pretty good and if you didn't know anything about Bandits, you probably wouldn't notice it was different... and might not notice if you do know your Bandits - unless it was parked next to one with a stock tank. IIRC, it upped the capacity `1-1.5 US gallons, so the end result would fit your requirements nicely.
As far as a bolt-on tank from something else, I doubt you'll find anything that'll fit and still look good.